A cloud is a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of a planet or other planetary body. A cloud is also a visible mass attracted by gravity (clouds can also occur as masses of material in interstellar space, where they are called interstellar clouds and nebulae).

The most common form of clouds on M-class planets are formed of water vapor. There are many classifications of clouds depending on their shape, density, altitude, and components. Low-level clouds that are very near or on a planet's surface are known as fog.

A common consequence of water vapor clouds on planets is the meteorological phenomenon known as rain. Another consequence of such clouds is lightning and thunder. Lightning or many other forms of electrical discharge in clouds is caused by the friction of the molecules moving inside the cloud.

Highly charged and dense clouds of water vapor, along with other atmospheric disturbances, can cause various types of storms.

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